1. Field of the Invention
The present invention relates to a roll paper conveying apparatus that conveys roll paper to a predetermined printing position, and also relates to an inkjet printer that uses the roll paper conveying apparatus.
2. Description of the Related Art
In a roll paper compatible printer that uses roll paper as a printing medium to print an image, the amount of the paper to be fed into a printing position must be controlled precisely because the load fluctuates as the amount of remaining roll paper being set changes. If the roll paper is directly pulled by carriage rollers, load fluctuation acts as a disturbance to the control of a conveying motor driving the carriage rollers, resulting in unstable control and therefore desired stop position precision may not be obtained. In addition, if roll paper having a large moment of inertia is directly pulled by the carriage rollers, the carriage rollers might slip on the roll paper to cause the amount of paper to be fed to change, resulting in print deviation even if the conveying motor is precisely controlled.
Therefore, such a roll paper compatible printer includes a driving source for unwinding the roll paper and pulling to covey the roll paper toward the carriage rollers. The driving source for unwinding and pulling the roll paper is provided separately from the conveying motor for driving the carriage rollers. Furthermore, a movable member is arranged upstream of the carriage roller so as to come into contact with the unwound roll paper and apply an optimal tension thereto. Thus, control of the conveying motor may be prevented from being affected adversely by a fluctuation in the load, which fluctuates as the amount of the remaining roll paper changes. Moreover, even if the roll paper slips, the amount of paper to be fed into the printing position can be controlled precisely.
In addition, as a technology related to conveyance of roll paper, Japanese Patent Application Laid-open No. S62-83968 discloses a technology that continuously detects the amount of displacement of a movable member, and increases or decreases the degree of roll paper to be unwound in a manner following the amount of displacement of the movable member to continuously manage pulling and conveying of the roll paper as well as tension thereof. More specifically, the technology disclosed in Japanese Patent Application Laid-open No. S62-83968 converts an encoder pulse corresponding to rotations of a roll paper driving motor for driving a roll paper shaft into a voltage, compares the voltage with a reference voltage, and performs proportional control to bring the difference between the voltage and the reference voltage to zero. The reference voltage is increased or decreased using the resistance of a rotary variable resistor corresponding to the roll paper diameter, and the resistance of a rotary variable resistor corresponding to the position of a movable guide plate.
The movable range of the movable member kept in contact with the roll paper to apply the optimal tension thereto is restricted by the space in which the movable member is installed inside the machine. In particular, when the machine is demanded to be compact, the movable range of the movable member tends to be restricted largely. In the technology disclosed in Japanese Patent Application Laid-open No. S62-83968, the rotations of the roll paper driving motor are controlled correspondingly to the amount of displacement of the movable member (movable guide plate). Thus, even if the amount of displacement of the movable member increases, the movable member is expected to gradually return to its home position (hereinafter, referred to as a “reference position”) as the roll paper driving motor is controlled continuously. However, such control lacks responsiveness. While the responsiveness of the control is expected to be improved by increasing the feedback gain, an increase in the feedback gain of the control of the roll paper driving motor that is to unwind and convey the roll paper having a large mass will result in oscillations and an unstable control. Therefore, the feedback gain cannot be increased.
Therefore, in an inkjet printer required to feed the roll paper intermittently in units of the print width to the printing position, it is difficult to bring the movable member to the reference position within a paper feeding cycle even if the technology disclosed in Japanese Patent Application Laid-open No. S62-83968 is applied in an attempt to control the amount of displacement of the movable member. A deviation of the movable member from the reference position gradually accumulates every time the intermittent paper feeding operation is repeated. As a result, the movable member might be displaced out of the movable range, and collide with the inner surface of the housing of the machine or other components inside of the machine, for example, to damage the movable member, to cause vibrations or abnormal noise, or to smear the paper.